There’s a bunch of variability in the answer to such a question.
TLDR;
A tri-fuel generator will cost you about $6400 in the first year and $800 per year after, assuming you’re doing a monthly trip and you need an 80% recharge.
A big battery + solar will cost you about $8800 one time.
A big battery-only solution will cost you about $8000 one time, assuming you need no more than 48% recharge at the far reach of your trip.
A tri-fuel generator doesn’t care, too much, about location, so it will generate what you need to generate most of the places you go. But it’s noisy and gasoline is messy and dangerous in random containers (much more dangerous than a battery).
A battery + solar solution will be less effective in some areas than others.
A battery-only solution is heavy, but doesn’t care about location and is silent.
We use solar+battery to give us an extra 30-60 miles during a camping trip, not to give us a full recharge. At least not if all we're bringing is the truck. If we had a travel trailer, I’d put 4kW of solar on the trailer and a big battery in the trailer. This would be a silent, quick solution and would give us unlimited overlanding distance in the US Southwest.
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You want an inverter generator, not a cheap open frame generator. This is because the EVSE (charger) you plug into it expects clean power. You also want an inverter generator that can charge your truck as quickly as possible because you don’t want it to have to run for 100+ hours. This means you want an inverter generator that can produce 240 V, 30+ amp (7.2 kW or more) for long periods of time, which usually means you need a generator rated for 15+ kW. You can get such a generator from a big box store for around $5k. Those can usually run on one of several fuels (gasoline, propane, or natural gas). The gasoline-fueled half-load runtime of the one I’m looking at at the big box store is 12 hours claimed, but that claim, in my experience, is usually an ideal situation claim, so I would estimate 10 hours maximum runtime, in good conditions. It claims it can do 50A, 240V, but my experience with generators is that you don’t want to pull the max amperage, so I would use the 30A output. Which is 7.2 kW.
Let’s say you've used 80% of your battery to get to your location and you need to charge to 100% so you can safely get to the nearest DCFC. On the Lightning, that’s 105 kWh. At 7.2 kW, that’s 15 hours. So you’ll need to run the generator to empty and then run it again for half a tank. The generator I’m looking at has a 13.2 gallon tank. So you’ll need about 20 gallons of gasoline (so bring four 5-gallon containers) to run your generator long enough to recharge the truck. If you run it on propane, you’ll get about 8.5 hours per 60 pounds. That’s three normal bbq-sized tanks. You would need 120 pounds, or 6 normal bbq-sized tanks of propane. But the generator manufacturer recommends a 60-lb tank minimum, so you’re likely going to use gasoline because we assumed no permanent installations at this site.
Gas in Virginia Beach is about $3/gallon right now, so that’s $60 of generator fuel per trip. Let’s say you do this trip once a month. That’s $720/year. Plus the cost of the generator ($5500), plus generator maintenance ($80-$120/year).
For the first year, that’s $5500+$840 = $6340.
For every year after, it’s about $800-$900 for a monthly trip to the boondocks.
You might be able to get away with a smaller generator, but you would have to listen to it for longer each run, you would have to do more regular maintenance on it, and you would use more gasoline. Plus if it’s a cheap open-frame generator, your EVSE may not always behave.
If I wanted a similar solution that is all-battery, I would need to bring a large battery. I can get a 62 kWh whole-home backup battery for about $8k, maybe less if I shop around, but let’s use $8k. That only provides about 48% of my recharge, so I also need to recharge it. I can install 800 Watts of solar on my truck without too much trouble and I could carry more with me. But let’s keep it to 800 Watts to see if that works. The retail cost of solar is about $0.40/Watt, so that’s about $320. Add in the mounting hardware, inverter, etc, and call it $800. Total setup cost is $8800.
After expending the 62 kWh battery to charge the truck, I would need an additional 43 kWh to bring the truck to 100%.
In Virginia Beach, the average daily energy from the sun is about 4.84 kWh per square meter. An 800 Watt system in Virginia Beach can generate about 3.9 kWh per day, on average. It would take—on average—about 11 days to generate enough to recharge that last 43 kWh. Obviously it would take longer in January than in July. If you double the PV size, which is unwieldy, you could cut that down to 5-6 days. Not great for a place like Virginia Beach.
In my neck of the woods, it would take around 4 days, on average, with an 800 Watt system.
There’s no ongoing maintenance cost to the solar system—it’ll last around 30 years if it’s taken care of. For a cost comparison, if you were able to make use of solar, then the solar+battery would pay for itself in about 3 years.
This assumes you need that full 80% charge. If you don’t need that big of a recharge, then things become easier more quickly. If, for example, you only needed 62 kWh and no solar, then you’d just need to pay the $8k for the battery, charge it at home before you go, load it into the vehicle, and recharge once you’re where you’re going. No noise. The cost to charge at home would be, for me, $0.00 because I would charge from solar. Before we got solar, it would be about $3.00.
Another complicating factor for the battery solution is weight.
The generator plus fuel is about 550 pounds. You can get a couple of friends to help you load the generator into the vehicle.
The battery is 1250 pounds. You’ll need some other solution to load the battery into the vehicle.